The multi-cylinder dryers of a papermachine have employed a so-called twin-wire run at the downstream end of a drying section. In the twin-wire run, the wire is supported against the jackets of cylinders included in two tiers of cylinders by means of two wires, one running along a tortuous path against the jackets of the cylinders of the upper tier and the other against the jackets of the cylinders of the lower tier. When passing over from one tier of cylinders to the other, the web travels unsupported. At machine speeds of more than 800 m/min, the air currents produced by the web and moving parts of the machine cause fluttering of the web in these free spaces. The fluttering leads to web breakups at the upstream end of such cylinder groups, as the strength properties of the web are still poor due to a high water content.
Efforts have been made to resolve this problem by means of a single-wire passage, wherein the free runs of the web are eliminated and the latter travels and is supported all the time by one and the same dryer wire between the cylinders included in two tiers. The drying effect of the cylinders of the same tier, whereat the wire lies between the web and the cylinder, is negligible as the wire prevents the transfer of heat from the cylinder to the web. Indeed, in the most recent machines, such cylinders have been replaced with suction rolls which has resulted in further improved machine operating characteristics and the threading ropes have become unnecessary.
Originally, the single-wire groups generally used to comprise just two or three first drive groups of a machine, but their number has been increased as the machine speeds have increased. Some recent machines completely lack the twin-wire cylinder groups. A drawback in a single-wire run is the increased length of the dryer section, leading to the increased length of the machine hall and, thus, to the increased factory building costs. On the other hand, a drawback affecting the grade of paper is that, in a single-wire run, heat is always supplied to a web from the same side of paper with possible defects developing in paper (curling).
As a summary of the above alternatives it can be said that the benefits of the twin-wire run include two-sided drying and a short dryer section but the drawbacks include poorer running characteristics at high machine speeds. The advantages and disadvantages of the single-wire run are essentially opposite relative to the above.
In addition, for example, U.S. Pat. No. 3,753,298 discloses a machine configuration, which employs a twin-wire run but in which the web is all the time supported by either one of the dryer wires. This is effected by passing the dryer wires by way of guide rolls mounted between the dryer cylinders in such a manner that, during the passage between a dryer cylinder and a guide roll, a dryer wire always in runs tangentially to the other guide roll and the other dryer wire wrapping therearound, whereby the web can be transferred from one dryer wire to another one at these points without free draws. A weakness of the solution disclosed in the cited publication is that the run of the web against the dryer wire between the dryer cylinders and wire guide rolls is not secured. Thus, the pressure differences prevailing in pockets defined by dryer cylinders and wire sections, the air currents produced thereby and, on the other hand, the adhesion forces between the web and cylinder surfaces detach the web from the dryer wire. Thus, the open, unsupported web is again susceptible to wrinkling and, at sufficiently high running speeds, this again leads to web breakups.
Naturally, attempts have been made to improve the operating characteristics of such a machine configuration by providing such multi-cylinder dryers at suitable locations with air-current controlling and/or producing structures. This type of solutions have been disclosed for example, in Finnish Patents 68279 (Patent application 841167) and 76142 (Patent application 854494). The passage of the web against the dryer wire is secured by using vacuum developing blow boxes. However, the blow box assemblies and nozzle designs proposed in the above references require quite large overall air quantities for a desired effect. These air quantities are typically about 2000-2400 m.sup.3 /hm per pocket. In terms of energy efficiency, this is undesired and leads to very large diameters in compensation air manifolds as well as highly sophisticated and expensive air circulation systems.
A design with a slightly different machine configuration from those described above is proposed in German patent 2,212,209, wherein open runs are eliminated by arranging each dryer wire to travel between the cylinders around a plurality of rolls, whereby the web always is transferred from a section, wherein it travels supported by the wire between a cylinder and the first wire guide roll, to a section wherein the second wire travels between the last wire guide roll and a cylinder included in the second tier. The transfer of the web from one dryer wire onto another is by no means secured and, especially at high machine speeds, the web may be disengaged from the dryer wire surface through the action of the above factors with the above-mentioned harmful consequences.